Fungal photobiology: visible light as a signal for stress, space and time

Abstract

Visible light is an important source of energy and information for much of life on this planet. Though fungi are neither photosynthetic nor capable of observing adjacent objects, it is estimated that the majority of fungal species display some form of light response, ranging from developmental decision-making to metabolic reprogramming to pathogenesis. As such, advances in our understanding of fungal photobiology will likely reach the broad fields impacted by these organisms, including agriculture, industry and medicine. In this review, we will first describe the mechanisms by which fungi sense light and then discuss the selective advantages likely imparted by their ability to do so.

Figure 1

A) Light induction of pigmentation. Incubation of Aspergillus fumigatus in constant white light leads to the production of blue-green melanin in the hyphae. Plate bottoms were photographed following 48 h incubation at 37°C. Deletion of the Neurospora wc-1 ortholog, lreA, abolishes this response (not shown) (Fuller et al. 2013). B) Light regulation of asexual development. Top: Light induces conidiation in a clinical A. fumigatus strain. Conidia were point-inoculated at the center of a petri plate and the top was photographed following incubation in an alternating light (12 h)/dark (12 h) environment for several days. The dark conidial bands correspond to the light-portion of the photocycle (white bars). Bottom: Light represses conidiation in a clinical Fusarium strain. Conidia were point-inoculated at the center of the petri-plate and the top was photographed following several days in an alternating light (12h)/dark (12h) environment that was interrupted by 24 h of constant light for one day (the wider white band). The lighter, fluffy bands correspond to the dark portions of the photocycle (black bars). The schematic proposes two models of developmental regulation by light in this Fusarium species, in which the WCC is presumed to be a major regulator. C) WCC in the Neurospora clock. In constant darkness, the WCC interacts with the c-box of the frq promoter and drives expression of frq gene. The FRQ protein interacts with FRH to negatively feedback on the WCC, thereby leading to reduced WCC activity and frq expression. Together with FRQ turnover, this feedback loop leads to observable oscillations in FRQ protein that are discernable when the protein is fused to the luciferase reporter. Beyond FRQ, the WCC also drives output from the clock as it regulates other genes whose products do not participate in the feedback loop (ccgs). Included among the ccgs are those involved in asexual development, resulting in the rhythmic conidiation pattern formed when the fungus grows across race tubes in constant darkness.